On burning or the like, for example in an automatic telephone exchange, plastic materials burn releasing fumes and products of pyrolysis containing particles in suspension and corrosive gases. When these hot gases come into contact with colder parts, of which the temperature is less than their dew point, and in particular with metallic conductors, they are deposited by condensation on these conductors. In numerous cases, these deposits contain elements such as halogen ions, acids, bases, which corrode the metals by a chemical process, and indeed an electro-chemical process where there exists electric voltage.
A device for measuring the corrosiveness of smoke permits then determination, by successive trials, of the plastic materials of which the combustion will cause the least damage to conductive parts.
There is already known in the prior art, from a number of publications of the applicants, a device for measuring corrosiveness of smoke comprising a chamber, means for creating in the chamber an atmosphere of air of predetermined humidity, heating means for heating the said atmosphere and the walls of the chamber to a predetermined temperature, means for causing combustion of the sample of material in the chamber, water circulation cooling means for condensing the smoke and products of pyrolysis generated nn the said combustion on a resistive metallic element exposed inside the chamber, the measurement of the corrosiveness of the smoke being carried out by measuring the variation of electrical resistance of the said element.
For allowing condensation of the smoke on a colder measurement test piece, constituted by the resistive element, the atmosphere inside the chamber should be maintained at a temperature of the order of 50.degree., which is provided, in the prior art, by using as a heating means a climatic chamber in which the device is placed.
However, an inconvenience of this solution is that it is then impossible to observe the behaviour of the device, and particularly the manner in which the combustion, the condensation of the combustion products on the test piece, and the attack of the metal proceeds.
In this regard, with such a known device, it is necessary in the embodiment of the chamber to find a difficult compromise between good resistance to aggresive gases, so as not to alter the characteristics of the atmosphere inside the chamber, and a relative thermal transparency. With this in mind, the chamber of the prior art device comprises a Pyrex (Registered trade mark) glass cylinder covered internally with a coating resistant to aggressive gases, but not transparent and threaded end walls of methyl polymethacrylate across which pass various lines associated with the means for carrying out the measurement.
Further, a a major parameter which must be controlled for ensuring reproducability of the measurement of the corrosion is the relative humidity inside the chamber. This humidity is affected by any loss of gas-tightness which leads, by re-establishment of equilibrium of the partial pressures between the internal humid atmosphere and the dry atmosphere of the climatic chamber, to a notable diminution of the proportion of water molecules involved in the condensation.
Further, the condensation of the combustion products on the test piece is disturbed by undesirable cooling of certain parts of the walls of the chamber, and an ill-determined proportion of the humid smoke present in the chamber is then condensed on these parts instead of condensing on the test piece.